The purpose of this study is to determine the effect of a 4-week treatment period with GW501516X on how the heart obtains and uses energy. The energy of the heart will be measured by Magnetic Resonance Imaging (MRI). This study will also measure a number of other potential markers of drug activity, including levels of certain lipids (fats) and proteins in your blood. The data from this study may help researchers better understand the actions of this drug in the body and if this drug may be useful to treat patients with heart disease.